The present invention relates to electronic depth meters which calculate the partial pressures of an inert gas in the tissues of a human body different in saturation half-time during diving and obtains data on non-decompression or decompression diving from the partial pressures of the inert gas in the tissues.
When a diver has stayed at a position deeper than a predetermined depth for more than a given time, he is required to be subjected to decompression which involves staying at a designated depth for a predetermined time so as to avoid suffering from a submarine sickness when ne ascends to the water surface.
In order to provide easy decompression, for example, U.S. Pat. No. 4,835,716 discloses an electronic depth meter which calculates a decompression stop depth and a decompression stop time from the deepest diving depth and the diving time using a table of decompression which is created by the U.S. Navy and displays these data digitally. Another electronic depth meter has been proposed which has the function of calculating and displaying the remaining time in which the diver can ascend without being subjected to decompression or the remaining time up to a non-decompression limit on the basis of the maximum depth and diving time.
If a diver wants to ascend without being subjected to decompression after he dived, he is required to recognize the remaining time up to a non-decompression limit using an electronic depth meter such as that mentioned above and to ascend before the remaining time before the non-decompression limit becomes 0. If the remaining time before the non-decompression limit has become 0, he must stop this ascent at a specified decompression stop depth, stay and be subjected to decompression at that decompression depth for a decompression stay time on the basis of data indicated by an electronic depth meter such as that mentioned above for ascending purposes.
Such electronic depth meter obtains decompression data on the assumption of the diver's stay throughout a diving interval of time at the maximum depth in consideration of maximum safety, so that the non-decompression limit is excessively short.
Under such situations, electronic depth meters have been invented which calculate respective quantities of nitrogen in the tissues of a human body on a real time basis and obtain and display data on appropriate decompression on the basis of those quantities of nitrogen, as disclosed in U.S. Pat. Nos. 4,005,282; 4,192,001; and 4,054,783. These depth meters classify the tissues of a human body according to saturation half-time (the time taken to reach 50% of a saturated quantity of nitrogen) in consideration of the rates of dissolution and discharge of nitrogen into and from the respective body tissues being different depending on the respective tissues, calculates the respective quantities of nitrogen using data on the saturation half-times, and obtain and display data on non-decompression diving or decompression diving on the basis of the respective quantities of nitrogen and safety limit quantities of nitrogen. Some of these devices display the shortest one of the respective remaining times up to the non-decompression limit for the tissues so as to indicate the non-decompression limit.
If the remaining times up to a non-decompression limit for tissues having short saturation half-times are small and the remaining times up to the non-decompression limit time for tissues having long saturation half-times are yet large, slight ascent serves to prolong the remaining times up to the non-decompression limit for the tissues having short saturation half-times, so that so careful attention is not required to be paid to such remaining times. However, if the remaining times up to the non-decompression limit for all the tissues are short, a danger is pressing and attention is required. The above depth meters only display the shortest remaining time up to the non-decompression limit in any one of two cases where the diver is required to pay careful attention and where the driver is not required to pay careful attention, so that he cannot recognize which of these two cases he is in.
In consideration of these situations, U.S. Pat. Nos. 4,782,338 and 5,049,864 have proposed simultaneous graphic display of quantities of nitrogen in the body tissues. According to this technique, it can be seen whether only a quantity of nitrogen for a specified tissue is large or all the quantities of nitrogen in the body tissues are large. However, the respective remaining times to the non-decompression limit cannot be known from the quantities of nitrogen. Therefore, the diver cannot know the extent to which the remaining time to the non-decompression limit is prolonged if the quantity of nitrogen in the most dangerous tissue is reduced, and the extend to which the remaining time before the non-decompression limit for that tissue is increased.
According to such electronic depth meters, the diver cannot directly know whether the current depth tends to increase or decrease the remaining time before the non-decompression limit, and he is required to wait until he can determine such situation by watching an increase or a decrease in the remaining time up to the non-decompression limit or the quantities of nitrogen actually displayed. Thus, this depth meter is inconvenient to use.
U.S. Pat. Nos. 4,782,338 and 5,049,864 disclose a technique for displaying as to whether the quantities of nitrogen in the body tissues have exceeded the limit value, using a mark indicative of a non-decompression limit value attached to a graphic indicator for the quantities of nitrogen in the body tissues, but it is difficult to understand to what extent the quantities of nitrogen in the tissues are close to, or exceed, the limit value.